In real life I am annoyed (not meaning road rage) by people who drive a lot slower (20–40 km/h) than the speed limit while the road conditions are perfect. It has really become a nuisance in my part of Germany over the last decade or so, and it was definitely not a thing way back when I got my license. I suspect the reasons are related to old age, fear of animals crossing or distraction by multimedia (phone, entertainment, loads of beeping and blinking »driving assistants« in modern cars), but I do not know for sure.

One thing I always enjoyed while playing ETS/ATS (I started around ETS2 1.19) was that the AI drivers in this regard were better than real life drivers and usually drove as fast as allowed. To my great disappointment SCS started to add more and more slow driving vehicles (not talking of tractors here) to AI traffic over the recent years which is a change for the worse in my opinion. If I remember correctly it started with »Vive la France« release.

I have no interest in creating and maintaining a public mod, but in this guide I will show you a few things that you can do to fix these issues. Please note that they will not work correctly if you use other traffic mods at the same time. Also be aware that the release of game version 1.37 might break this fix.

Bran Dragon has put the changes from this guide into a mod. Maybe it works for you:
https://drive.google.com/open?id=1Oiv22_xhbT6iT93FWzJcni-NLje2ef0S
https://steamproxy.net/sharedfiles/filedetails/?id=2064402515
Please note that this will not work correctly if you use other traffic mods at the same time.

First you have to extract the file def.scs using the SCS Extractor.

Then go to the folder

Edit the .sii files for cars, other small vehicles and their trailers. They look like this:
and so on.

Some of them have a max speed value that is too low.
Check every small vehicle for the following line(s) and edit their max speed value(s) (in km/h) if it is too low for your liking:

Be aware that some files contain more than one vehicle and therefore more than one max value, so do scroll down while checking/editing.

Ignore the files for trains, »crashed« vehicles, »event« vehicles and rescue vehicles.

Some trucks aren't able to accelerate fast enough after spawning in front of you (not talking of steep hills). This is caused by low torque values. The torque values chosen by the developers may be technically correct, but they are unfortunately too low for an immersive gameplay experience considering the way the game engine simulates traffic.

To fix this, edit the truck .sii files. They look like this:
and so on.

Check all truck files and change their torque value to 3500:

The buses have a weak engine. The only way I have found so far to fix this is decreasing their weight.

Open both bus files:

Edit their mass values:

Open the file
and add the lines
This apparently tells the AI to go no more than 10 % over the speed limit.

Open the file
and edit the line
This apparently tells the AI not to drive slower when it's raining.

To get all the fixes into the game, you have to put all edited files (except the config.cfg change) into your own little mod (or a subfolder* of your mod folder) and activate it in the mod manager.

* Example for using subfolder instead of .zip file:

Of course it is important to preserve the folder structure while packing or moving the files.

The changes in this guide are working very nicely for me, but I am in no way responsible for any problems that you might encounter. Use at your own risk.

Thank you for your interest and your feedback.
Happy trucking!